Method for adding boron to metal alloys

ABSTRACT

A method to grain refine and deoxidize a precious metal alloy or a master alloy includes the steps of (a) forming a precursor melt consisting essentially of constituents of the precious metal alloy or master alloy and inevitable impurities; (b) dispersing a compound selected from the group consisting of boron containing metal hydrides, boron containing metal fluorides and mixtures thereof throughout the precursor melt; and (c) solidifying the boron containing precious melt alloy or master alloy. One suitable compound is solid sodium borohydride (sodium tetrahydroborate). To minimize evaporation of the boron on contact with the precursor alloy melt, the sodium borohydride may be wrapped in a metal foil formed from constituents of the precious metal alloy or master alloy. The cast precious metal alloy or master alloy has been found to have a reduced number of hard spots and reduced silicon contamination when compared to conventional casting methods.

CROSS REFERENCE TO RELATED APPLICATION(S)

This patent application claims priority to U.S. provisional patentapplication Ser. No. 60/672,566 by P. G. Johns and S. A. Davis that wasfiled on Apr. 19, 2005; United States non-provisional patent applicationSer. No. 11/132,621 by S. A. Davis, N. D. Baker, J. J. Riskalla and R.V. Carrano that was filed on May 19, 2005; and U.S. provisional patentapplication Ser. No. 60/732,784 by P. G. Johns and S. A. Davis that wasfiled on Nov. 2, 2005. The subject matters of all three U.S. patentapplication Ser. Nos. 60/672,566; 11/132,621 and 60/732,784 areincorporated by reference herein in their entireties.

U.S. GOVERNMENT RIGHTS

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process to manufacture boron containingprecious metal alloys and master alloys. More particularly a solidcompound that is either a boron containing metal hydride, and preferablya solid tetrahydroborate, or a boron containing metal fluoride isdispersed throughout a molten precious metal alloy or master alloy.

2. Description of the Related Art

Precious metal jewelry alloys are frequently worked into complexornamental shapes. To withstand extensive working without fracture, theprecious metal alloys require high ductility and high strength. Highductility and high strength are facilitated by an alloy having a lowoxygen content and a fine grain structure.

Boron is known to both deoxidize and refine the grain of precious metalalloys. When boron scavenges oxygen from a melt and other oxides in themelt, it cleanses surfaces of the metal. U.S. Pat. No. 5,384,089 toDiamond discloses the use of boron as a deoxidizer for gold-base alloys.This patent discloses that boron causes hard spots. U.S. Pat. No.6,168,071 to Johns discloses a diffusion bondablesilver-copper-germanium alloy that may contain up to 20 parts permillion of boron as a grain refiner. The boron is disclosed as added asa component of a copper-2%, by weight, boron master alloy. Both U.S.Pat. No. 5,384,089 and U.S. Pat. No. 6,168,071 are incorporated byreference herein in their entireties. Throughout this patentapplication, all percentages are weight percent, unless otherwisespecified.

A conventional method of introducing boron into a precious metal alloyor master alloy is through the use of the 98% copper-2% boron masteralloy. However, the use of such a master alloy frequently introduceshard spots into the products. These hard spots are believed to benon-equilibrium phase CuB₂₂ particles that form in copper saturated withboron when cooled from the liquid phase to the solid phase. Hard spotscan also form with other metal-boride compounds such as iron borides(for example Fe₅B₂ and FeB₂). The hard spots are frequently not detecteduntil after the precious metal jewelry alloy is polished and inspectedresulting in needless expense for the processing of ultimatelyunsatisfactory product.

Copper—2% boron master alloys are frequently contaminated with silicon.The silicon contamination may lead to brittleness, as a result of theformation of brittle intermetallic compounds, oxides and low meltingeutectics. A low silicon content is required for fine, white gold andfine silver.

A further disadvantage with the use of a copper—2% boron master alloy isthat the high mass percent of copper may not be desired for the alloyproduct. Excess copper may cause a silver-base alloy to be subject totarnish and/or firestain.

There remains, therefore, a need for a more effective way to introduceboron as a grain refiner and oxygen/oxide scavenger into a preciousmetal melt.

BRIEF SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a method to producea precious metal alloy or master alloy. This method includes the stepsof (a) forming a molten precursor alloy of the precious metal alloy ormaster alloy, (b) disbursing a boron containing compound throughout themolten precursor alloy, and (c) solidifying the boron containingprecursor alloy.

It is feature of the invention that the boron containing compound iseither a boron containing metal hydride or a boron containing metalfluoride. When a boron containing metal hydride, the metal may besodium, lithium, potassium, calcium, zinc and mixtures thereof. When aboron containing metal fluoride, the metal is sodium. Most preferred asa boron containing compound is sodium boro-hydride.

It is another feature of the invention that more than 20 ppm of boroncan be incorporated into a silver-base, or other precious metal, alloywithout the development of hard spots.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in flow chart representation an initial processingsequence for the alloys of the invention.

FIG. 2 illustrates in flow chart representation subsequent processing ofthe alloys of the invention in accordance with a first embodiment of theinvention.

FIG. 3 illustrates in flow chart representation subsequent processing ofthe alloys of the invention in accordance with a second embodiment ofthe invention.

FIG. 4 graphically illustrates the rate of boron loss in a batch processof the invention.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

The following definitions are used throughout this patent application:

Master Alloy—Constituents of a precious metal alloy omitting thepredominant precious metal. For example, a yellow 10, 14 or 18 karatalloy may contain both silver and gold, only the gold would be omittedin the master alloy. Silver would be present. For a sterling silveralloy, the silver would be omitted and there would be no precious metalconstituent present. Master alloys are usually sent to an end user whoadds the required amount of precious metal.

Precious Metal Alloy—An alloy having a desired composition for jewelryapplications. The alloy includes required amounts of gold, silver,palladium and/or platinum.

Precursor Alloy—A composition slightly off specification for a desiredmaster alloy or precious metal alloy. The addition of a metal foilcontaining boron compound places the composition on specification. Ifthe boron compound is not wrapped in metal foil, for example wrapped inpaper or not wrapped, the precursor alloy composition is onspecification for the desired master alloy or precious metal alloy.

The process of the invention is useful to add boron to precious metalalloys and to master alloys with a minimal formation of hard spots.Exemplary of the precious metal alloys are sterling silver alloys andsilver alloys containing in excess of 75% silver with the balance beingalloying elements, including, but not limited to, copper and zinc, andinevitable impurities. Silver alloys and sterling silver alloys havingbetween 80% and 97% silver are most benefited by the process of theinvention.

The process is also useful for gold jewelry alloys having at least 33%,by weight, of gold (8 karat) with the balance being alloying elementsincluding, but not limited to silver, nickel, copper and zinc as well asinevitable impurities. Most benefited by the process of the inventionare those gold alloys having between 37.5% and 77% gold.

FIG. 1 illustrates in flow chart representation an initial processingsequence of the alloys of the invention. A precursor melt of theprecious metal alloy or master alloy is formed by melting 10 appropriateamounts of the precious metal and alloying elements in a suitablecrucible. As described below, a boron containing compound may be wrappedin a metallic foil formed from either the precious metal or one of thealloying elements and added to the precursor melt. Accordingly, theadditional metal content of the foil is taken into consideration and thecomposition of the precursor melt is typically slightly different thanthe composition of the desired end product.

The alloy is melted 10 in a suitable crucible. For silver alloys, onesuitable crucible is formed from clay-graphite and for gold alloys onesuitable crucible is ceramic. Other suitable crucibles for silver-baseand gold-base alloys include clay-graphite, fused silica, siliconcarbide, graphite and zirconia. The metals are heated to a temperatureeffective to fully liquify and flow the mixture, typically in the rangeof from 1950° F. to 2300° F., with a nominal temperature on the order of2150° F. The melting temperature influences the kinetics of boronevaporation which determines the final boron concentration in the castprecious metal alloy or master alloy. The selected temperature should besufficiently above the liquidous temperature of the alloy to preventfreezing in a die during continuous casting or freezing in a grain boxduring grain making. While the alloys are readily cast at atmosphericpressures, higher or lower pressures should not affect the benefits ofthe invention, but will affect the kinetics of boron evaporation.

To reduce the formation of an oxide slag, the molten precursor alloyshould be covered to isolate the metal surface from oxygen. Suitable gascovers include, but are not limited to, a carbon monoxide flame, forminggas flame, argon, nitrogen, hydrogen flame and natural gas flame.Suitable powdered solid covers include, but are not limited to, borax,boric acid, graphite and charcoal.

Once the precursor melt is at the desired molten temperature, a boroncontaining compound is added 12 to the precursor melt. Boron isincorporated into precious metal alloys as an oxygen scavenger and, forsilver alloys, additionally or alternatively as a grain refiner. It maybe added to molten silver by bubbling a gaseous borane, e.g. diboraneinto the alloy in admixture with a non-reactive gas such as argon, byintroducing into the alloy a borane which is solid at ambienttemperature, e.g. decaborane B₁₀H₁₄ (melting point=100° C., boilingpoint=213° C.), or by adding an alkylated borane, e.g. triethylborane ortri-n-butyl borane, although the latter reagents are spontaneouslycombustible and require care in handling.

When added to the precious metal in the gas phase, the boron compound isadvantageously an admixture with a carrier gas that assists in creatinga stirring action in the molten alloy and dispersing the boron contentof the gas mixture into the alloy. Suitable carrier gases include,hydrogen, nitrogen and argon. The gaseous boron compound and the carriergas may be introduced into a vessel containing molten silver orsilver-base alloy by using a metallurgical lance that may be anelongated tubular body of refractory material, e.g. graphite, or it maybe a metal tube clad in refractory material. The lance is preferably ofsufficient length to permit injection of the gaseous boron compound andcarrier gas deep within the molten alloy.

Alternatively, the boron-containing gas may be introduced into themolten alloy from a side or bottom of the alloy-containing vesselthrough a gas transport member, such as a gas permeable bubbling plug ora submerged injection nozzle. Rautomead International of Dundee,Scotland, manufactures horizontal continuous casting machines in the RMKseries for the continuous casting of semi-finished silver-base andgold-base products. The alloy to be heated is placed in a solid graphitecrucible, protected by an inert gas atmosphere which may, for example,be oxygen-free nitrogen containing <5 ppm oxygen and <2 ppm moisture andis heated by electrical resistance heating using graphite blocks. Suchfurnaces have a built-in facility for bubbling inert gas through themelt. Addition of small quantities of thermally decomposableboron-containing gas to the inert gas being bubbled through the meltreadily provides a desired few ppm or few tens of ppm boron content. Theintroduction of the boron compound into the alloy as a dilute gas streamover a period of time, the carrier gas of the gas stream serving to stirthe molten metal or alloy, rather than in one or more relatively largequantities is believed to be favorable from the standpoint of avoidingdevelopment in the metal or alloy of boron hard spots.

Compounds which may be introduced into molten silver or gold or alloysthereof as a gas include boron trifluoride, diborane or trimethylboronwhich are available in pressurized cylinders diluted with hydrogen,argon, nitrogen or helium, diborane being preferred because apart fromthe boron, the only other element is introduced into the alloy ishydrogen. A yet further possibility is to bubble carrier gas through themolten silver to effect stirring thereof and to add a solid boroncompound e.g. NaBH₄ or NaBF₄ into the fluidized gas stream as a finelydivided powder which forms an aerosol.

The boron compound may also be introduced into the molten silver or goldalloy in the liquid phase, either as such or in an inert organicsolvent. Compounds which may be introduced in this way includealkylboranes or alkoxy-alkyl boranes such as triethylborane,tripropylborane, tri-n-butylborane and methoxydiethylborane which forsafe handling may be dissolved in hexane or tetrahydrofuran (THF). Theliquid boron compound may be filled and sealed into containers ofsilver-base or of copper-base foil resembling a capsule or sachet usingknown liquid/capsule or liquid/sachet filling machinery and using aprotective atmosphere to give filled capsules sachets or other smallcontainers typically of capacity 0.5-5 ml, more typically about 1-1.5ml. As an alternative, especially for gold casting, the capsules orsachets may be of a polymer film, e.g. polyethylene or polypropylene.The filled capsules or sachets in appropriate number may then be plungedindividually or as one or more groups into the molten silver or gold oralloy thereof. A yet further possibility is to atomize the liquidboron-containing compound into a stream of carrier gas which is used tostir the molten silver as described above. The droplets may take theform of an aerosol in the carrier gas stream, or they may becomevaporized therein.

Preferably, the boron is added as a metal borohydride, e.g. aborohydride of an alkali metal, a pseudo-alkali metal or an alkalineearth metal, e.g. lithium borohydride. Sodium borohydride is especiallypreferred because it is widely commercially available and can beobtained in the form of relatively large pellets that are convenient tohandle during precious metal melting operations.

The boron is advantageously solid, e.g a metal borohydride or a higherborane, such as decaborane, and is in the form of pellets or granuleswhich are advantageously wrapped in a layer of foil of precious metaland plunged as a group into the molten metal. Boron can be added to theother molten components both on first melting and at intervals duringcasting to make up for boron loss if the alloy is held in the moltenstate for a period of time, such as in a continuous casting process forgrain. Addition of boron to a molten copper-base master alloy is notrecommended because adding boron changes the copper content and hencethe overall proportions of the various constituents in the alloy.

The boron is added in the form of either a boron containing metalhydride, and preferably as a solid tetrahydroborate, or a boroncontaining metal fluoride. When a boron containing metal hydride,suitable metals include sodium, lithium, potassium, calcium, zinc andmixtures thereof. When a boron containing metal fluoride, sodium is thepreferred metal. Most preferred is sodium borohydride, NaBH₄, alsoreferred to as sodium tetrahydroborate. Sodium borohydride has amolecular weight of 37.85 and contains 28.75% boron.

Sufficient boron is added so that an effective amount remains in thecast precious metal alloy or master alloy for effective grain refinementand deoxidation. Between 1 ppm and 1600 ppm boron remaining iseffective. Preferably, the boron content is between 100 ppm and 1600 ppmfor a master alloy and between 1 ppm and 1000 ppm for a silver- orgold-base precious metal alloy. A nominal boron content in the castprecious metal alloy or master alloy of about 250 ppm is most preferred.Typically, from 0.001% to 0.16% of boron added to the precursor alloymelt is effective.

Boron reacts to form a gas that evaporates at elevated temperatures andit may be necessary to make sequential additions of boron as describedhereinbelow to maintain an adequate concentration for grain refining. Toenable better mixing into the precursor alloy, the boron compound may bewrapped in a thin metallic foil. The foil may be any constituent of themaster melt or an inert material, such as paper, and is preferably aductile metal that may be formed into a relatively thin foil. Preferredmetals for the foil include silver, copper and gold. The foil has athickness of from about 0.01 millimeter to about 0.3 millimeter toenable the foil wrapped boron compound to be well submerged in themaster melt before the foil melts through releasing the boron compound.Once released, the constituents of the boron compound combine withoxygen in the precursor melt to effectively deoxidize the melt and theboron reacts with some of the elements in the melt to form discreteinsoluble particles dispersed throughout the base material which act asnucleation sites promoting the formation of fine grains that are uniformin size and resist growth.

When sodium borohydride is first added to the master melt, the initialreaction is believed to be decomposition of the boron containing grainrefiner.NaBH_(4(s))→Na_((g))+B_((s))+2H_(2(g))  (1)When diborane is first added to the molten metal, the decompositionreaction is believed to be:B₂H₆→3B_((s))+3H_(2(g))  (2)The hydrogen is effective to deoxidize the melt.

After decomposition, the sodium, hydrogen and boron are all effective todeoxidize the melt as follows:Na_((g))+0.5O_(2(g))→Na₂O_((s))  (3)H_(2(g))+0.5O_(2(g))→H₂O_((g))  (4)B_((s))+0.5O_(2(g))+0.5H_(2(g))→HBO_((g))  (5)

To achieve a uniform casting, the boron is dispersed throughout theprecursor melt by stirring 14. Preferably, the boron is stirred 14 forin excess of 1 minute and typically for from 1-5 minutes. Stirring maybe by any means which does not contaminate the precursor melt such aswith a graphite stirring rod.

The molten precious metal alloy or master alloy is then cast 16 by amethod suitable for forming a desired end product.

One such useful end product is casting grains. Casting grains areroughly spherical particles which are sold to jewelry manufacturers whothen investment cast to form a desired article of jewelry. Subsequent tostirring 14, molten precious metal alloy is poured into a grain box 18,FIG. 2. A grain box is a container with openings in the bottom, throughwhich the liquid metal flows to make the desired shape and size ofgrains. The grain box is made from materials similar to the crucible,such as, but not limited to, graphite, clay/graphite, ceramic andsilicon carbide. The molten precious metal alloy is formed into discretedroplets in the grain box as it flows through the openings and is thensolidified into roughly spherical particles in grain tank 20. A graintank 20 contains water into which the droplets fall and solidify.

The particles are then removed from the grain tank 20 and dried 22 bycentrifugal force and hot air. The roughly spherical grains have atypical diameter of from about 0.1 millimeter to about 5 mm.

In accordance with a second embodiment of the invention, continuouscasting may be used to form wrought mill products such as sheet, tubingand wire that is later made into finished products such as jewelry. Thestirred boron containing molten metal alloy is transferred to a die 24,FIG. 3, and partially solidified in the die such that a cohesivestructure may be withdrawn from the die and subjected to secondarycooling to 26 such as by impact with water spray or passing through achilled coil. The continuous cast structure is then finished 28 such asby passing through rolling mills and shears to achieve a desiredcross-sectional shape and surface finish and then coiled 30 for shipmentto jewelry manufacturers. Continuous cast sheet has a more consistentboron content than achieved by prior art processes which enables moreconsistent welding into tubing.

For a well stirred batch process, as illustrated in the combination ofFIGS. 1 and 2, the concentration of boron decreases over time inaccordance with the equationC _(B) =C _(B,0)exp(−kρt/m)  (5)Where C_(B)=the present boron content in ppm.C_(B,0)=the initial boron content in ppm.k=a rate constant dependent on alloy composition, temperature, gas coverand melt cover expressed in units of inch³/minute.ρ=the density of the alloy in troy ounces/inch³.t=time in minutes.m=melt weight in troy ounces.

Equation (5) predicts that the rate of boron evaporation is faster forsmaller melt sizes and this was observed in practice. FIG. 4 illustratesin graphical representation the kinetics of boron loss in the batchmelting process in the exemplary condition of a CO gas cover and agraphite powder solid cover.

For a continuous casting process as illustrated in the combination ofFIGS. 1 and 3, the material balance must take into account the change inmass in the casting crucible with time. The amount of boron present maybe calculated by the equation:C _(B) =C _(B,0)(m ₀/(m ₀ −F ₀ t))exp−(kρ/F ₀)  (6)Where C_(B), C_(B,0), t, k and ρ were previously defined.m₀=the initial mass (in troy ounces) of alloy in a crucible at time=0.F₀=the casting rate in troy ounces/minute.

The time, t, is reset to zero and the initial alloy mass m₀, isrecalculated after each incremental addition of boron.

The above-described process may be used to make alloys of any preciousmetal, especially fine silver, fine gold and alloys thereof that arerequired to incorporate boron. In particular, it may be used to makesilver/germanium alloys having an Ag content of at least 77%, by weight,a Ge content of between 0.5 and 3%, by weight, an amount of boroneffective to enhance ductility and strength and the balance copper alongwith incidental ingredients and impurities. If desired, the germaniumcontent may be substituted, in part, by one or more incidentalingredient elements selected from Al, Ba, Be, Cd, Co, Cr, Er, Ga, In,Mg, Mn, Ni, Pb, Pd, Pt, Si, Sn, Ti, V, Y, Yb and Zr, provided that theeffect of germanium in providing firestain and tarnish resistance is notunduly affected. The weight ratio of germanium to incidental ingredientelements may range from 100:0 to 60:40, and preferably ranges from 100:0to 80:0. The term “incidental ingredient” permits the ingredient to haveancillary functionality within the alloy, e.g. to improve color oras-molded appearance and includes the metals or metalloids Si, Zn, Snand In in amounts appropriate for deoxidation. The invention is alsoapplicable for the manufacture of mater alloys, such as Cu/Ge/B and CuB.

The alloys that may be made according to the process of the inventioninclude coinage grade, 800-grade (80% by weight silver) (including 830and 850 grades (83% and 85%, by weight, silver, respectively) and thelike) and standard Sterling silver and an alloy of silver containing anamount of germanium effective to reduce firestain and/or tarnishing. Theternary Ag—Cu—Ge alloys and quaternary Ag—Cu—Zn—Ge alloys that cansuitably be made are those having a minimum silver content of 80%, byweight, and more preferably, a minimum silver content, by weight, of92.5%. The maximum silver content is 98%, by weight, and preferably, themaximum silver content is 97%, by weight. The germanium content is atleast 0.1%, and preferably at least 0.5%, more preferably at least 1.1%,and most preferably at least 1.5%, by weight. The maximum germaniumcontent is preferably 6.5% and more preferably 4.0%, by weight.

Silicon may be added to silver alloys in an amount of up to 5%, byweight, preferably from 0.5 to 3 weight percent and most preferably inan amount of from 0.1 to 0.2 weight percent. When incorporated intocasting grain of an Ag—Cu—Ge ternary alloy, it can provide brightinvestment casting immediately on removal from the mold. It may be addedto casting grain, e.g. before investment casting, or it may beincorporated into the silver at the time of first melting to form analloy.

The process of the invention may be used to manufacture gold jewelryalloys having at least 33%, by weight, of gold (8 karat) with thebalance being alloying elements including, but not limited to silver,nickel, copper and zinc, as well as inevitable impurities. Particulargold alloys that may be processed in accord with the invention include:

24K gold—a minimum of 99.7%, by weight, of gold with the balance beinggrain refiners, hardening additives and impurities.

22K gold—exemplary is, by weight, 91.67% Au, 5% Ag, 2% Cu and 1.33% Zn.

18K gold—exemplary are, by weight, 75% Au, 20% Ag and 5% Cu; 75% Au, 15%Ag and 10% Cu; 75% Au, 13% Ag and 12% Cu; 75% Au, 5% Ag and 20% Cu; 75%Au, 2.75% Ag and 22.25% Cu; 75% Au and 25% Cu; 80% Au and 20% Al; 75%Au, 25% Pt, Pd or Ag; 75% Au, 10% Pd, 10% Ni and 5% Zn; 75% Au, 17% Feand 8% Cu and 75% Au, 23% Cu and 2% Cd.

14K gold—exemplary are, by weight, 58.33% Au, 24,78% Cu and 0.14% Zn;58.33% Au, 4.00% Ag, 31.24% Cu, 6.43% Zn, 0.10% Ni, 0.05% Fe; and 0.01%Si and 58.33% Au, 2.08% Ag and 39.59% Cu.

10K gold—exemplary are, by weight, 41.70% Au, 11.66% Ag, 40.81% Cu,5.83% Zn, 0.03% Si and 0.02% B; 41.70% Au, 5.50% Ag, 43.80% Cu and 9.00%Zn; and 41.70% Ag, 2.82% Ag and 55.48% Cu.

The above-described invention is better understood by the examples whichfollow:

EXAMPLES Example 1 (Prophetic)—Ag—Cu—Ge—Si Alloy

A silver alloy is made by melting together 93.2 weight percent finesilver casting grains, 1.3 weight percent germanium in the form of smallbroken pieces, 0.2 weight percent silicon (added as a Cu/Si master alloycontaining 10 weight percent silicon) and the balance being coppergranules. Melting is by means of a gas-fired furnace heated to a pourtemperature of about 2000° F. The melt is covered with graphite toprotect against atmospheric oxidation. In addition, a hydrogen gasprotective flame is provided. Stirring is by hand using a graphitestirring rod.

When the alloy constituents liquefy, 20.2 grams (0.65 troy ounce) ofsodium borohydride per 46.7 kilogram (1500 oz.) melt are wrapped in apure silver foil, about 0.15 mm thick. The foil wrapper holds the sodiumborohydride to prevent it from floating to the surface of the melt. Thewrapped sodium borohydride is placed into a hollow cup-shaped end of agraphite stirring rod and plunged beneath the surface of the melt. Themelt is covered with a ceramic fiber blanket to quench a flame resultingfrom decomposition of the borohydride. The hydrogen and sodium burns offwith a bright yellow flame over a period of 1-2 minutes during whichtime the melt is continuously stirred. When the evolution of hydrogenceases, the boron is substantially incorporated into the melt togetherwith at least some sodium.

After the boron is added, the crucible is pivoted to permit pouring themolten alloy into a tundish having a bottom formed with very fine holes.The molten silver flows into the tundish and through the holes in finestreams that break into fine pellets and fall into a stirred water bathbecoming solidified and cooled. The cast pellets are then removed fromthe bath and dried.

The pellets are tested by investment casting using a calcium sulphatebonded investment. The resulting casting has a matte silvery finish whenremoved from the mold, a fine grain structure, and can be easilypolished. It is free from boron hard spots and was ductile as exhibitedby a capability to make a ring stretch 4-6 sizes whereas a similarmaterial made using copper/boron can only be stretched about two sizes.

Example 2 (Working)—Manufacture of Sterling Silver Casting Grain

Two hundred troy ounces of a sterling silver precursor melt were meltedin a clay-graphite crucible. The precursor melt had a nominalcomposition, by weight, of 93% silver, 5.7% copper and 1.3% germanium.The precursor melt constituents were mixed together and heated under acarbon monoxide flame and covered with a one inch thick layer of boraxsalt. When the precursor melt temperature reached the flow temperature,0.0125% boron was added as NaBH₄. The boron compound was wrapped in 0.15mm silver foil for introduction to the master melt. Sufficient power wasprovided to maintain the temperature of the molten precious metal alloyat the flow temperature. The molten precious metal alloy was thenstirred with a graphite stirring rod for 3.7 minutes and poured into agrain box. The molten precious metal alloy was protected by a reducingatmosphere during pouring at the flow temperature. After about 0.25minutes, the entire molten precious metal alloy was converted intocasting grains.

The casting grains were assayed and found to have 13.8 ppm boron. Thegrains were mounted, polished and etched for examination of grainstructure and hard spots. The resulting grain structure was fine andcontained no boron hard spots. The material was not brittle when reduced75% by thickness in a rolling mill. Investment cast rings formed fromthe casting grains contained no fire scale or hard spots. The rings werestretched 3.25 sizes without annealing before failure.

Example 3 (Working)—Manufacture of Sterling Silver Grains by BatchProcess

Table 1 illustrates that the process of the invention is effective toadd boron to a sterling silver precursor alloy and that the melt coverappears to have more of an effect on the boron content in the preciousmetal alloy than does the pour gas. In no instance were hard spotsdetected on the cast grains. TABLE 1 Boron Time Content Between BoronAmount in Boron Content Melt NaBH₄ Pour Precursor Addition in Cast MeltSize Added Pour Melt Temp. Alloy and Pour Grains Number (troyoz) (grams)Gas Cover (° F.) (ppm) (minutes) (ppm) 0281 200 2.78 N₂/H₂ Borax 2150125 3.7 13.8 0322 1500 3.33 N₂/H₂ Borax 2150 20 4.0 9.5 0326 200 3.04 COBorax 2150 137 4.25 23.5 0327 200 3.04 CO Borax 2150 137 6.5 6.1 0338/3400 2.02 CO Borax 2150 45.5 5.0 23.9 0353 1500 5.05 N₂/H₂ Borax 215030.3 5.25 12.2 0339 72 8.09 CO Borax 2150 1011 3.0 9.0 081006  300 3.03CO Borax 2150 91 4.0 11.4 B01 200 17.77 CO Graphite 2000 800 5.0 0.6 B02200 8.89 CO Graphite 2000 400 5.0 0.6 B03 200 5.55 CO Graphite 2000 2505.0 1.3 BS4 200 35.53 CO Graphite 2200 1600 6.4 2.4 BS05 200 8.89 COGraphite 2200 400 5.0 0.5 0605 1000 20.2 N₂/H₂ Borax 2150 182 5 42.90610 1000 20.2 N₂/H₂ Borax 2150 182 5 59.9 0611 1000 20.2 N₂/H₂ Borax2150 182 5 85.7

Example 4 (Working)—Manufacture of Sterling Silver Continuous CastProducts

4500 troy ounces of a sterling silver precursor melt were melted in aclay-graphite crucible. The master melt had a nominal composition, byweight, of 93% silver, 5.7% copper and 1.3% germanium. The precursormelt constituents were mixed together and heated under a natural gasflame and covered with a layer of charcoal. When the precursor melttemperature reached the flow temperature, 0.0020% boron was added asNaBH₄. The boron compound was wrapped in 0.15 mm silver foil forintroduction to the precursor melt. Sufficient power was provided tomaintain the temperature of the molten precious metal alloy at the flowtemperature. The NaBH₄ was added incrementally to maintain a good boronconcentration by taking into account the change in melt weight and theevaporation of boron with time. A timer was used to add the boron asscheduled below. Each boron addition was placed inside a graphiteplunger and mixed into the molten precious metal alloy at the timesindicated in Table 2. TABLE 2 Time From Transfer Boron Added as NaBH₄,(minutes) (dry weight grams) 0 6.50 6 5.85 12 5.20 18 4.55 24 3.9 303.25 36 2.60 42 1.95 48 1.30 54 0.65

At transfer, to the continuous casting die, the molten precious metalalloy was heated to the transfer temperature and cast into a continuoustwo inch diameter cylindrical bar at the casting temperature under anatural gas flame and charcoal cover. Samples of the cast precious metalalloy were assayed for boron level. Table 3 summarizes the assayresults. TABLE 3 Time (min.) Boron Concentration (ppm) 8.5 2.9 17 1.825.48 2.5 34 1.0 42.5 1.4

Example 5 (Working)—Manufacture of 18 Karat White Gold Casting Grain

125 troy ounces of a white gold precious metal alloy were melted in aceramic crucible. The precious metal alloy melt had a nominalcomposition of 75% gold and the balance 6% nickel, 14% copper and 5%zinc. The precious metal alloy constituents were melted together andheated under a carbon monoxide flame to the flow temperature at whichtime 0.08% boron as NaBH was added. The boron compound was wrapped in apaper envelope for introduction to the melt. Sufficient power wasprovided to maintain the temperature of the molten precious metal alloyat the flow temperature. The molten precious metal alloy was stirred for1.75 minutes subsequent to the boron addition and then poured into agrain box. The gas cover during pour into the grain box was a reducingatmosphere. After about 0.25 minutes, the entire molten precious metalalloy was converted into casting grain. Analysis of the casting grainshowed a clean surface, no hard spots and a fine grain size.

One or more embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1. A method to cast a precious metal alloy or master alloy, comprisingthe steps of: (a). forming a precursor alloy melt consisting essentiallyof constituents of said precious metal alloy or said master alloy andinevitable impurities; (b). dispersing a compound selected from thegroup consisting of boron containing metal hydrides, boron containingmetal fluorides and mixtures thereof throughout said master melt; and(c). solidifying said boron containing precious melt alloy or masteralloy.
 2. The method of claim 1 wherein said metal constituent of saidboron containing metal hydride is selected from the group consisting ofsodium, lithium, potassium, calcium, zinc and mixtures thereof and saidmetal constituent of said boron containing metal fluoride is sodium. 3.The method of claim 2 wherein said compound is selected to be solidsodium borohydride (sodium tetrahydroborate).
 4. The method of claim 2wherein said boron containing metal hydride or said boron containingmetal fluoride is wrapped in a metal foil selected to be one of saidconstituents of said precious metal alloy or master alloy prior to beingdispersed in said precursor alloy melt.
 5. The method of claim 4 whereinsaid metal foil is selected to have a thickness of between 0.01millimeter and 0.3 millimeter.
 6. The method of claim 4 wherein saidprecious metal alloy or master alloy contains silver and said metal foilis selected to be silver or a silver-base alloy.
 7. The method of claim4 wherein said precious metal alloy contains gold and said metal foil isselected to be copper or a copper-base alloy.
 8. The method of claim 4wherein said dispersing step (b) includes stirring for a time effectiveto disperse boron throughout said precious metal alloy or said masteralloy.
 9. The method of claim 1 wherein said precious metal alloy ormaster alloy is transferred to a grain box.
 10. The method of claim 1wherein said precious metal alloy is transferred to a continuous castingdie and withdrawn following said solidifying step (c) as an extendedlength of desired cross-sectional shape.
 11. The method of claim 10wherein said dispersing step (b) is repeated multiple times to maintaina desired boron content.
 12. The method of claim 4 wherein sufficientboron is added to obtain a precious metal alloy or master alloy having,by weight, from 1 ppm to 1600 ppm of boron.
 13. The method of claim 12wherein said boron content, by weight, is from 100 ppm to 1600 ppm forsaid master alloy and from 1 ppm to 100 ppm for said precious metalalloy.
 14. A silver- or gold-base alloy or master alloy containing, byweight, from 1 ppm to 1600 ppm of boron and being substantially free ofboth silicon and copper.
 15. The silver- or gold-base alloy of claim 14wherein said boron content is from 100 ppm to 1600 ppm for said masteralloy and from 1 ppm to 100 ppm for said precious metal alloy. 16.Casting grain formed from the silver- or gold-base alloy or master alloyof claim
 14. 17. Casting grain having a nominal composition, by weight,selected from the group consisting of 93% silver, 5.7% copper and 1.3%germanium; 74.8% gold, 12.2% nickel; 9.9% copper and 3.1% zinc; and81.4% copper and 18.6% germanium, all plus inevitable impurities.
 18. Anextended length of desired cross-sectional area formed from the silver-or gold-base alloy of claim
 14. 19. The extended length of claim 18formed from an alloy having a nominal composition by weight of 93%silver, 5.7% copper, 1.3% germanium and inevitable impurities.
 20. Theextended length of claim 18 being a sheet formed into a productedselected from the group consisting of wire, chain, sheet and weldedtube.
 21. A method to cast a precious metal alloy or master alloy,comprising the steps of: (a). forming a molten alloy in a crucible, saidmolten alloy consisting essentially of constituents of said preciousmetal alloy or said master alloy and inevitable impurities; (b).bubbling a boron containing gas through said molten alloy melt; and (c).solidifying said boron containing molten alloy.
 22. The method of claim21 wherein said boron containing gas is mixed with a carrier gasselected from the group consisting of hydrogen, nitrogen, argon, heliumand mixtures and compounds thereof.
 23. The method of claim 22 whereinsaid boron containing gas is selected from the group consisting ofdiborane, boron trifluoride and trimethyl boron.
 24. The method of claim22 wherein said boron containing gas is an aerosol containing dispersedboron-containing particles.
 25. The method of claim 24 wherein saidboron-containing particles are selected from the group consisting ofNaBH₄ and NaBF₄.
 26. The method of claim 22 including the step ofinserting a lance into said molten alloy and flowing said boroncontaining gas through said lance.
 27. The method of claim 22 includingforming said crucible with a gas transporting member and flowing saidboron containing gas through said gas transporting member.
 28. Castinggrain selected from the group consisting silver-base and gold-basealloys formed by the method of claim
 22. 29. An extended length of adesired cross-sectional area selected from the group consisting ofsilver-base and gold-base alloys formed by the method of claim
 22. 30. Amethod to cast a precious metal alloy or master alloy, comprising thesteps of: (a). forming a precursor alloy melt consisting essentially ofconstituents of said precious metal alloy or said master alloy andinevitable impurities; (b). introducing a boron containing liquid tosaid precursor alloy melt; and (c). solidifying said boron containingprecious melt alloy or master alloy.
 31. The method of claim 30including selecting said boron containing liquid from the groupconsisting of alkylboranes, alkyoxy-alkyl boranes, triethylborane,tripropylborane, tri-n-butylborane, methoxydiethylborane and mixturesthereof.
 32. The method of claim 31 including mixing said boroncontaining liquid with an organic solvent.
 33. The method of claim 32wherein said organic solvent is selected from the group consisting ofhexane, tetrahydrofuran and mixtures thereof.
 34. The method of claim 31wherein said boron containing liquid is encapsulated.
 35. The method ofclaim 34 wherein said boron containing liquid is encapsulated in amaterial selected from the group consisting of silver-base foil,copper-base foil and polymer film.
 36. The method of claim 31 whereindroplets of said boron containing liquid are combined with a carrier gasand introduced to said molten alloy melt as an aerosol.
 37. Castinggrain selected from the group consisting silver-base and gold-basealloys formed by the method of claim
 31. 38. An extended length of adesired cross-sectional area selected from the group consisting ofsilver-base and gold-base alloys formed by the method of claim 31.